Method for the secondary cross-linking of hydrogels with 2-oxotetrahydro-1,3-oxazines

ABSTRACT

Liquid-absorbent polymers are prepared by a process for the gel and/or surface postcrosslinking of water-absorbent polymers by the polymer being treated with a surface postcrosslinking solution and being postcrosslinked and dried during and after the treatment by raising the temperature, wherein the crosslinker comprises a compound of the formula I                    
     where R 1  is hydrogen, C 1 -C 4 -alkyl, C 1 -C 4 -hydroxyalkyl, trialkylsilyl or acetyl and R 2 , R 2′ , R 3 , R 3′ , R 4 , R 4′  are each independently hydrogen, C 1 -C 12 -alkyl, C 2 -C 12 -alkenyl or C 6 -C 12 -aryl, dissolved in an inert solvent, and are used in hygiene articles, packaging materials and nonwovens.

DESCRIPTION

The present invention relates to a process for the gel or surfacepostcrosslinking of water-absorbent hydrogels with2-oxotetrahydro-1,3-oxazines, the polymers thus obtainable and their usein hygiene articles, packaging materials and nonwovens.

Hydrophilic, highly swellable hydrogels are in particular polymers of(co)polymerized hydrophilic monomers, graft (co)polymers of one or morehydrophilic monomers on a suitable grafting base, crosslinked celluloseor starch ethers, crosslinked carboxymethylcellulose, partly crosslinkedpolyalkylene oxide or natural products that are swellable in aqueousfluids, for example guar derivatives. Such hydrogels are used asproducts for absorbing aqueous solutions in the manufacture of diapers,tampons, sanitary napkins and other hygiene articles, and as waterretainers in market gardening.

To improve application properties, for example diaper rewet andabsorbency under load (AUL), hydrophilic, highly swellable hydrogels aregenerally surface or gel postcrosslinked. This postcrosslinking ispreferably carried out in the aqueous gel phase or as surfacepostcrosslinking of the ground and classified polymer particles.

Useful crosslinkers for this purpose include compounds containing atleast two groups capable of entering covalent bonds with the carboxylgroups of the hydrophilic polymer. Useful compounds include for exampledi- or polyglycidyl compounds, such as diglycidyl phosphonate,alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines,and these compounds can also be used in mixtures with each other (seefor example EP-A-0 083 022, EP-A-0 543 303 and EP-A-0 530 438).Polyamidoamines useful as crosslinkers are described in EP-A-0 349 935in particular.

A major disadvantage of these crosslinkers is their high reactivity,since it necessitates particular precautions in production to avoidundesirable side effects. Moreover, the aforementioned crosslinkers haveskin-irritating properties, which makes their use in hygiene articlesproblematical.

Known crosslinkers also include polyfunctional alcohols. For instance,EP-A-0 372 981, U.S. Pat. No. 4,666,983, and U.S. Pat. No. 5,385,983teach the use of hydrophilic polyalcohols and the use of polyhydroxysurfactants. The reaction is carried out at 120-250° C. The process hasthe disadvantage that the esterification which leads to crosslinking isvery slow even at such temperatures.

Prior German Patent Application DE-A-19 807 502 describes a process forpostcrosslinking with 2-oxazolidinones.

It is an object of the present invention to provide gel or surfacepostcrosslinking equivalent to or superior to the prior art by usingrelatively inert compounds capable of reacting with carboxyl groups.This object is to be achieved with a very short reaction time and a verylow reaction temperature.

We have found that this object is achieved, surprisingly, when2-oxotetrahydro-1,3-oxazines are used as crosslinkers. Moreparticularly, the moderate reactivity of the crosslinkers can be boostedwith inorganic or organic acidic catalysts. Useful catalysts includeknown inorganic mineral acids, their acidic salts with alkali metals orammonium and also their corresponding anhydrides. Useful organiccatalysts include known carboxylic acids, sulfonic acids and aminoacids.

The invention accordingly provides a process for the gel and/or surfacepostcrosslinking of water-absorbent polymers by the polymer beingtreated with a surface postcrosslinking solution and beingpostcrosslinked and dried during and after the treatment by raising thetemperature, wherein the crosslinker comprises a compound of the formulaI

where R¹ is hydrogen, C₁-C₄-alkyl, C₁-C₄-hydroxyalkyl, trialkylsilyl oracetyl and R², R^(2′), R³, R^(3′), R⁴, R^(4′) are each independentlyhydrogen, C₁-C₁₂-alkyl, C₁-C₁₂-alkenyl or C₆-C₁₂-aryl, dissolved in aninert solvent.

The postcrosslinking and drying temperature is preferably 50-250° C.,especially 50-200° C., most preferably 100-180° C. The surfacepostcrosslinking solution is preferably sprayed onto the polymer insuitable spray mixers. Following spray application, the polymer powderis dried thermally, and the crosslinking reaction can take place notonly before but also during the drying. Preference is given to sprayapplication of a solution of the crosslinker in reaction mixers ormixing and drying systems such as, for example, Lödige mixers, BEPEX®mixers, NAUTA® mixers, SHUGGI® mixers or PROCESSALL®. Moreover,fluidized-bed dryers may also be used.

Drying may take place in the mixer itself, by heating the outer casingor by blowing hot air in. It is similarly possible to use a downstreamdryer such as a tray dryer, a rotary tube dryer or a heatable screw. Butit is also possible, for example, to use an azeotropic distillation as adrying technique. The preferred residence time at this temperature inthe reaction mixer or dryer is less than 60 min, particularly preferablyless than 30 min.

In a preferred embodiment of the invention, the reaction is acceleratedby adding an acidic catalyst to the surface postcrosslinking solution.Useful catalysts for the process of the invention include all inorganicacids, their corresponding anhydrides, and organic acids. Examples areboric acid, sulfuric acid, hydroiodic acid, phosphoric acid, tartaricacid, acetic acid and toluenesulfonic acid. More particularly theirpolymeric forms, anhydrides and also the acidic salts of the polybasicacids are also suitable. Examples of these are boron oxide, sulfurtrioxide, diphosphorus pentoxide and ammonium dihydrogenphosphate.

The crosslinker is dissolved in inert solvents. The crosslinker is usedin an amount of from 0.01 to 5%, preferably 0.01-1.0%, preferably from0.05 to 0.5%, by weight, based on the polymer used. The preferred inertsolvent is water or a mixture of water with mono- or polyhydricalcohols. However, it is also possible to use any unlimitedlywater-miscible organic solvent which is not itself reactive under theprocess conditions. When an alcohol-water mixture is used, the alcoholcontent of this solution is for example 10-90% by weight, preferably30-70% by weight, especially 40-60% by weight. Any alcohol of unlimitedmiscibility with water can be used, as can mixtures of two or morealcohols (eg. methanol+glycerol+water). The alcohol mixtures may containthe alcohols in any desired mixing ratio. However, it is particularlypreferable to use the following alcohols in aqueous solution: methanol,ethanol, isopropanol, ethylene glycol and particularly preferably1,2-propanediol and 1,3-propanediol.

In a further preferred embodiment of the invention, the surfacepostcrosslinking solution is used in a ratio of 1-20% by weight, basedon the mass of the polymer. Particular preference is given to a solutionquantity of 0.5-10% by weight, based on the polymer.

The invention further provides crosslinked water-absorbent polymers thatare obtainable by the process according to the invention.

The hydrophilic, highly swellable hydrogels to be used in the process ofthe invention are in particular polymers of (co)polymerized hydrophilicmonomers, graft (co)polymers of one or more hydrophilic monomers on asuitable grafting base, crosslinked cellulose or starch ethers ornatural products swellable in aqueous fluids, for example guarderivatives. Preferably the polymer to be crosslinked is a polymercontaining sstructural units derived from acrylic acid or its esters, orobtained by graft copolymerization of acrylic acid or acrylic estersonto a water-soluble polymer matrix. These hydrogels are known to oneskilled in the art and are described for example in U.S. Pat. No. 4,286,082, DE-C-27 06 135, U.S. Pat. No. 4,340,706, DE-C-37 13 601,DE-C-28 40 010, DE-A-43 44 548, DE-A-40 20 780, DE-A-40 15 085, DE-A-3917 846, DE-A-38 07 289, DE-A-35 33 337, DE-A-35 03 458, DE-A-42 44 548,DE-A-42 19 607, DE-A-40 21 847, DE-A-38 31 261, DE-A-35 11 086, DE-A-3118 172, DE-A-30 28 043, DE-A-44 18 881, EP-A-0 801 483, EP-A-0 455 985,EP-A-0 467 073, EP-A-0 312 952, EP-A-0 205 874, EP-A-0 499 774, DE-A 2612 846, DE-A-40 20 780, EP-A-0 205 674, U.S. Pat. No. 5,145,906, EP-A-0530 438, EP-A-0 670 073, U.S. Pat. No. 4,057,521, U.S Pat. No.4,062,817, U.S. Pat. No. 4,525,527, U.S. Pat. No. 4,295,987, U.S. Pat.No. 5,011,892, U.S. Pat. No. 4,076,663 or U.S. Pat. No. 4,931,497. Thecontent of the aforementioned patent documents is expressly incorporatedherein by reference.

Examples of hydrophilic monomers useful for preparing these hydrophilic,highly swellable hydrogels are polymerizable acids, such as acrylicacid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, maleicacid including its anhydride, fumaric acid, itaconic acid,2-acrylamido-2-methylpropanesulfonic acid,2-acrylamido-2-methylpropanephosphonic acid and its amides, hydroxyalkylesters and amino- or ammonium-containing esters and amides and also thealkali metal and/or ammonium salts of monomers containing acid groups.Also suitable are water-soluble N-vinylamides such as N-vinylformamideor else diallyldimethyl-ammonium chloride. Preferred hydrophilicmonomers are compounds of the general formula II

where

R⁵ is hydrogen, methyl or ethyl,

R⁶ is —COOR⁸, hydroxysulfonyl or phosphonyl, a(C₁-C₄)-alkanol-esterified phosphonyl group or a group of the formulaIII

R⁷ is hydrogen, methyl, ethyl or carboxyl,

R⁸ is hydrogen, amino-(C₁-C₄)-alkyl, hydroxy-(C₁-C₄)-alkyl, alkali metalor ammonium ion and

R⁹ is a sulfonyl group, a phosphonyl group or a carboxyl group or analkali metal or ammonium salt of each of these.

Examples of C₁-C₄-alkanols are methanol, ethanol, n-propanol,isopropanol or n-butanol.

Particularly preferred hydrophilic monomers are acrylic acid andmethacrylic acid and also their alkali metal and ammonium salts, forexample sodium acrylate, potassium acrylate or ammonium acrylate.

Useful grafting bases for hydrophilic hydrogels obtainable by graftcopolymerization of olefinically unsaturated acids or their alkali metalor ammonium salts may be of natural or synthetic origin. Examples arestarch, cellulose or cellulose derivatives and also otherpolysaccharides and oligosaccharides, polyalkylene oxides, in particularpolyethylene oxides and polyethylene oxides and polypropylene oxides,and also hydrophilic polyesters.

Useful polyalkylene oxides have for example the formula IV

where

R¹⁰ and R¹¹ are independently hydrogen, alkyl, alkenyl or aryl,

X is hydrogen or methyl, and

n is an integer from 1 to 10,000.

R¹⁰ and R¹¹ are each preferably hydrogen, (C₁-C₄)alkyl, (C₂-C₆)alkenylor phenyl.

Preferred hydrogels are in particular polyacrylates, polymethacrylatesand also the graft polymers described in U.S. Pat. No. 4,931,497, U.S.Pat. No. 5,011,892 and U.S. Pat. No. 5,041,496.

The hydrophilic, highly swellable hydrogels are preferably incrosslinked form; that is, they include compounds having at least twodouble bonds which have been copolymerized into the polymer network.Suitable crosslinkers are in particular N,N′-methylenebisacrylamide,N,N′-methylenebismethacrylamide, esters of unsaturated mono- orpolycarboxylic acids of polyols, such as diacrylate or triacrylate,examples being the diacrylates and dimethacrylates of butanediol and ofethylene glycol, and trimethylolpropanetriacrylate, and also allylcompounds such as allyl (meth)acrylate, triallyl cyanurate, diallylmaleate, polyallyl esters, tetraallyloxyethane, triallylamine,tetraallylethylenediamine, allyl esters of phosphoric acid and alsovinylphosphonic acid derivatives as described for example in EP-A-0 343427. In the process of the invention, however, particular preference isgiven to hydrogels prepared using polyallyl ethers as crosslinkers andby acidic homopolymerization of acrylic acid. Suitable crosslinkers arepentaerythritol tri- and tetraallyl ether, polyethylene glycol diallylether, monoethylene glycol diallyl ether, glycerol di- and triallylether, polyallyl ethers based on sorbitol and also ethoxylated variantsthereof.

The water-absorbent polymer is preferably a polymeric acrylic acid or apolyacrylate. This water-absorbent polymer may be prepared by a processknown from the literature. Preference is given to polymers containingcrosslinking comonomers in amounts of 0.001-10 mol %, preferably 0.01-1mol %, but very particular preference is given to polymers obtained byfree-radical polymerization using a polyfunctional ethylenicallyunsaturated free-radical crosslinker which additionally bears at leastone free hydroxyl group (eg. pentaerythritol triallyl ether ortrimethylolpropane diallyl ether).

The hydrophilic, highly swellable hydrogels are preparable byconventional polymerization processes. Preference is given to additionpolymerization in aqueous solution by the process known as gelpolymerization. In this process from 15 to 50% strength by weightaqueous solutions of one or more hydrophilic monomers and optionally ofa suitable grafting base are polymerized in the presence of afree-radical initiator, preferably without mechanical mixing, utilizingthe Trommsdorff-Norrish effect (Makromol. Chem. 1, 169 (1947)). Thepolymerization reaction may be carried out in the temperature range from0 to 150° C., preferably from 10 to 100° C., not only at atmosphericpressure but also at elevated or reduced pressure. As customary, thepolymerization may also be carried out in a protective gas atmosphere,preferably under nitrogen. The polymerization may be initiated usinghigh-energy electromagnetic radiation or the customary chemicalpolymerization initiators, for example organic peroxides, such asbenzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketoneperoxide, cumene hydroperoxide, azo compounds such asazodiisobutyronitrile and also inorganic peroxy compounds such as(NH₄)₂S₂O₈. K₂S₂O₈ or H₂O₂. They may if desired be used in combinationwith reducing agents such as sodium hydrogensulfite and iron(II) sulfateor redox systems where the reducing component is an aliphatic oraromatic sulfinic acid, such as benzenesulfinic acid or toluenesulfinicacid or derivatives thereof, such as Mannich adducts of sulfinic acids,aldehydes and amino compounds as described in DE-A-1 301 566. Thequalities of the polymers may be further improved by postheating thepolymer gels for a number of hours within the temperature range from 50to 130° C., preferably from 70 to 100° C.

The gels obtained are neutralized for example to the extent of 0-100 mol%, preferably 25-100 mol %, particularly preferably 50-85 mol %, basedon monomer used, for which the customary neutralizing agent can be used,preferably alkali metal hydroxides or oxides, but particularlypreferably sodium hydroxide, sodium carbonate or sodium bicarbonate.

Neutralization is customarily effected by mixing in the neutralizingagent as an aqueous solution or else, preferably, as a solid. For thispurpose the gel is mechanically.comminuted, by means of a mincer forexample, and the neutralizing agent is sprayed on, scattered over orpoured on and then carefully mixed in. To effect homogenization, theresultant gel mass may be passed through the mincer again a number oftimes. The neutralized gel mass is then dried with a belt dryer orroller dryer until the residual moisture content is less than 10% byweight, especially below 5% by weight. The dried hydrogel is then groundand sieved, the customary grinding apparatus being roll mills, pin millsor vibratory mills. The particle size of the sieved hydrogel ispreferably in the range 45-1000 μm, particularly preferably 45-850 μmmost preferably 200-850 μm.

To ascertain the quality of surface postcrosslinking, the dried hydrogelis tested using the test methods described hereinbelow:

Methods

1) Centrifuge Retention Capacity (CRC):

This method measures the free swellability of the hydrogel in a teabag.About 0.200 g of dried hydrogel is sealed in a teabag (format: 60 mm×60mm, Dexter 1234 T paper) and soaked for 30 min in a 0.9% strength byweight sodium chloride solution. The teabag is then spun for 3 min in acustomary commercial spindryer (Bauknecht WS 130, 1400 rpm, basketdiameter 230 mm). The amount of liquid absorbed is determined byweighing the centrifuged teabag. The absorption capacity of the teabagitself is taken into account by determining a blank value (teabagwithout hydrogel), which is deducted from the weighing result (teabagwith swollen hydrogel).

Retention CRC [g/g]=(weighing result for teabag−blank value−initialweight of hydrogel)÷initial weight of hydrogel.

2) Absorbency Under Load (0.3/0.5/0.7 psi):

For the absorbency under load, 0.900 g of dry hydrogel is distributeduniformly on the screen base of a measuring cell. The measuring cellconsists of a Plexiglass cylinder (50 mm in height and 60 mm indiameter) whose base is formed by adhering a screen of steel mesh (meshsize 36 micron or 400 mesh). A coverplate is placed over the uniformlydistributed hydrogel and loaded with an appropriate weight. The cell isthen placed on a filter paper (S&S 589 Schwarzband, diameter=90 mm)lying on a porous glass filter plate, this filter plate itself lying ina Petri dish (30 mm in height, 200 mm in diameter) which contains 0.9%strength by weight sodium chloride solution so that the liquid level atthe beginning of the experiment is level with the top edge of the glassfrit. Hydrogel is then left to absorb the salt solution for 60 min.Subsequently the complete cell with the swollen gel is removed from thefilter plate and the apparatus is reweighed following removal of theweight.

Absorbency under load (AUL) is calculated as follows:

AUL[g/g]=(Wb−Wa)÷Ws

where

Wb is the mass of the apparatus+gel after swelling,

Wa is the mass of the apparatus+initial weight of the gel

before swelling, and

Ws is the initial weight of dry hydrogel.

The apparatus is measuring cylinder+coverplate.

EXAMPLES 1A AND 1B

Base Polymer:

In a 40 l plastic bucket, 6.9 kg of glacial acrylic acid are dilutedwith 23 kg of water. 45 g of pentaerythritol triallyl ether are added tothis solution with stirring, and the sealed bucket is inertized bypassing nitrogen through it. The polymerization is then initiated byadding about 400 mg of hydrogen peroxide and 200 mg of ascorbic acid.After the reaction has ended, the gel is mechanically comminuted andadmixed with sufficient aqueous sodium hydroxide solution to provide adegree of neutralization of 75 mol %, based on the acrylic acid used.The neutralized gel is then dried on a roll dryer, ground with a pinmill and finally classified. This is the base polymer used in thesubsequent examples.

The base polymer is sprayed in a Waring lab blender with crosslinkersolution of the following composition: 4% by weight of methanol, 6% byweight of water and 0.20% by weight of 2-oxotetrahydro-1,3-oxazine,based on polymer used. The moist polymer is then divided into twoportions which are each heat treated at 175° C. in a through circulationcabinet, one portion for 60 min and the other for 90 min. The driedproduct is classified at 850 micron to remove lumps.

EXAMPLES 2a AND 2b

Base polymer as per Example 1 is sprayed with crosslinker solution in aWaring lab blender. The solution has a composition such that thefollowing dosage is obtained, based on base polymer used: 0.40% byweight of 2-oxotetrahydro-1,3-oxazine, 4% by weight of propylene glycoland 6% by weight of water. One portion of the moist polymer is thendried at 165° C. for 60 min, another at 165° C. for 90 min.

EXAMPLE 3

Base polymer as per Example 1 is sprayed with crosslinker solution in aWaring lab blender. The composition of the solution is such that thefollowing dosage is achieved, based on base polymer used: 0.30% byweight of 2-oxotetrahydro-1,3-oxazine, 3% by weight of 1,2-propanediol,7% by weight of water and 0.2% by weight of boric acid. The moistpolymer is then dried at 175° C. for 60 min.

EXAMPLE 4

Base polymer as per Example 1 is sprayed with crosslinker solution in aWaring lab blender. The composition of the solution is such that thefollowing dosage is achieved, based on base polymer used: 0.40% byweight of N-methyl-2-oxotetrahydro-1,3-oxazine, 4% by weight of ethanol,6% by weight of water and 0.2% by weight of ammoniumdihydrogenphosphate. The moist polymer is then dried at 175° C. for 60min.

The polymers prepared as per the above examples were tested. The resultsare reported below in Table 1.

TABLE 1 AUL AUL 0.7 psi Drying Drying CRC 0.3 psi (4826.5 Pa) temp. timeCrosslinker Catalyst Solvent (g/g) (g/g) (g/g) Example 1 — — — — — 42 109 Base polymer without surface crosslinking Example 1a 175° C. 60 min0.20% — 4% MeOH + 6% H₂O 40 31 9 Crosslinker 1 Example 1b 175° C. 90 min0.20% — 4% MeOH + 6% H₂O 38 33 13 Crosslinker 1 Example 2a 165° C. 60min 0.40% — 4% PG + 6% H₂O 36 35 18 Crosslinker 1 Example 2b 165° C. 90min 0.40% — 4% PG + 6% H₂O 32 33 24 Crosslinker 1 Example 3 175° C. 60min 0.30% 0.2% 3% PG + 7% H₂O 35 34 20 Crosslinker 1 H₃BO₃ Example 4175° C. 60 min 0.40% 0.2% 4% EtOH + 6% H₂O 31 32 25 Crosslinker 2NH₄H₂PO₄

Crosslinker 1: 2-oxotetrahydro-1,3-oxazine

Crosslinker 2: N-methyl-2-oxotetrahydro-1,3-oxazine

Percentages are by weight based on polymer used. Drying temperature andtime relate to the heat treatment of the base polymer after it has beensprayed with surface postcrosslinking solution.

We claim:
 1. A process for the gel or surface postcrosslinking ofwater-absorbing polymers, comprising treating polymers with a surfacepostcrosslinking solution containing a crosslinker, and postcrosslinkingand drying by increasing the temperature, wherein the crosslinkercomprises a compound of the formula I

wherein R¹ is hydrogen, C₁-C₄-alkyl, C₁-C₄-hydroxyalkyl, trialkylsilylor acetyl; R², R^(2′), R³, R^(3′), R⁴, R^(4′) are each independentlyhydrogen, C₁-C₁₂-alkyl, C₁-C₁₂-alkenyl or C₆-C₁₂-aryl, dissolved in aninert solvent.
 2. The process of claim 1, wherein the water-absorbingpolymer is selected from the group consisting of a polymeric acrylicacid, polymeric acrylic esters, graft copolymer of acrylic acid and awater-soluble polymer matrix, and a graft copolymer of acrylic estersand a water-soluble polymer matrix.
 3. The process of claim 1, whichfurther comprises a catalyst comprising an acid or an anhydride thereofwhen the polymers are treated with the surface postcrosslinkingsolution.
 4. The process of claim 3, wherein the catalyst is selectedfrom the group consisting of boric acid; sulfuric acid; hydroiodic acid;phosphoric acid; tartaric acid; acetic acid; toluenesulfonic acid; anacidic salt thereof; a polymeric form of boric acid; a polymeric form ofsulfuric acid; a polymeric form of phosphoric acid; and an anhydride ofboric acid, sulfuric acid, phosphoric acid, tartaric acid, acetic acid,or toluenesulfonic acid.
 5. The process of claim 1, wherein the inertsolvent is water, a mixture of water with monohydric alcohol is from 10to 90% by weight of the alcohol, or a mixture of water with a polyhydricalcohol is from 10 to 90% by weight of the alcohol.
 6. The process ofclaim 1, wherein the crosslinker is in an amount of from 0.01 to 5% byweight, based on the mass of the water-absorbing polymer.
 7. The processof claim 1, wherein the crosslinker is in an amount of from 0.01 to 1%by weight, based on the mass of the water-absorbing polymer.
 8. Theprocess of claim 1, wherein the crosslinker is in an amount of from 0.05to 0.5% by weight, based on the mass of the water-absorbing polymner. 9.The process of claim 1, wherein the postlinking and drying temperatureis from 50 to 250° C.
 10. The process of claim 1, wherein thepostlinking and drying temperature is from 50 to 200° C.
 11. The processof claim 1, wherein the postlinking and drying temperature is from 100to 180° C.
 12. The process of claim 1, wherein the crosslinker is2-oxotetrahydro-1,3-oxazine.
 13. The process of claim 1, wherein thecrosslinker is N-methyl-2-oxo-tetra hydro-1,3-oxazine.
 14. The processof claim 5, wherein the monohydric or polyhydric alcohol is selectedfrom.the group consisting of methanol, ethanol, isopropanol, ethyleneglycol, 1,2-propanediol, and 1,3-propanediol.
 15. The process of claim1, wherein the postcrosslinking solution is in an amount of from 1 to20% by weight based on the mass of the water-absorbing polymer.
 16. Theprocess of claims 1, wherein the postcrosslinking solution is in anamount of from 0.5 to 10% by weight based on the mass of thewater-absorbing polymer.